The present invention relates to a flowmeter proving apparatus for proving flowmeters having non-uniform pulse outputs by using a reference volume cylinder and more particularly to a flowmeter proving apparatus having the function of calculating the number of proving runs necessary for obtaining the required repeatability of the proving test by using a small volume prover.
There are two conventional methods for proving flowmeters, one of which is called "an absolute proving method" whereby a flowmeter to be proved is connected in series with a precision cylinder having a calibrated reference volume and its reading during the displacement of the fluid of the reference volume through the calibrated section of the cylinder is directly compared to the above-mentioned fluid's volume or weight, and the other is called "a comparison method" whereby the flowmeter's reading is compared with a standard flowmeter. The "absolute proving method" is applied in the case where a high accuracy of instrumental error-correction is required. Generally, the absolute proving method is classified into two methods; one is "the tank method" using a standard tank having the known volume of its portion defined by its upper and lower levels, and the other is "the pipe method" using a reference volume pipe having the known volume in its calibrated section of the uniform cross-section.
The "standard tank method" cannot attain a high efficiency in performing proving tests since it requires much time and labor for reading the upper and lower levels, calculating the reference volume from the readings and so on. Conversely, in the case of "the pipe prover method", since whole meter pulses generated by a flowmeter to be proved are counted, and a rubber-made sphere of a diameter a little larger than the reference volume pipe's inside diameter is movable in the pipe under a low differential pressure of liquid therein, travels through the pipe section between its two positioned detectors (hereinafter called detectors). The result is compared with the reference volume and it is possible to obtain remote control and automated measurements thereby allowing the rationalization of the proving operation.
As regards the development of a flowmeter having a higher accuracy and a diversification of fluids for which flowmeters are used, it is necessary to achieve real time proving of the flowmeters through the pipe prover method using a small volume prover (hereinafter abbreviated to SVP) which is distinguished by its reduced size thereby allowing it to be transported by car or other vehicle by virtue of its short and small reference volume pipe.
The operating principle of SVP is such that a piston, sealably and slidably inserted in a cylinder of an even cross-section, travels through a calibrated section of the cylinder with its limits defined by the detectors to displace a constant volume of fluid through a flowmeter to be proved by comparing its readings with the measured volume of the fluid.
SVP systems are disclosed in chapter 4 "Proving System" of The Manual of Petroleum Measurement Standards issued in June of 1988 by The American Petroleum Institute.
As mentioned above, the flowmeter proving method by using the SVP is meant to compare the fluid's volume in the calibrated section of the reference volume pipe defined by the signals emitted from the detectors with the number of meter pulses emitted from the flowmeter for the same period. The time-interval between the first detector signal at proving pass start and the first meter pulse following said detector signal and the time-interval between the last detector signal at the proving pass end and the meter pulse proceeding or following said detector signal, i.e. volume less than meter pulse spacing, are determined as ratios to the number of high frequency clock pulses and a fractional part of the displaced volume are calculated as a sum of or the difference between the ratios (by the double-timing method).
However, the double-timing method requires the conducting of the proving at a constant flow-rate and the generation of meter pulses at a uniform pulse spacing, that is, if the flow-rate varies or the pulses are emitted in non-uniform pulse spacing, the result may involve a corresponding error.
The flowmeter's pulse space dispersion depend upon the types of flowmeters to be proved. For example, a turbine meter, in which a rotor, rotatable in proportion to the flow-rate to be measured, is placed close to the flowmeter sensor, may generate equally spaced pulses of an excellent SN-ratio while pulse dispersion may occur in the case of a flowmeter having a rotary transmitting mechanism such as a gear train between a rotor and a meter pulse generator as well as a positive displacement flowmeter wherein a rotor's rotation angle is not proportional to the flowmeter's displacement.
The above-mentioned manual of API proposes that in case of a flowmeter having a rotor placed adjacent to the meter pulse generator, the proving test shall be conducted five times (by five proving passes) in order to attain a meter repeatability of 0.05% and a meter factor (in liters per pulse) shall be calculated as an average of values obtained by these five tests.
In a proving test of a flowmeter, generating irregularly spaced pulses representing certain flow-rates, it is necessary to increase the number of a piston's travels (hereinafter called the number of proving passes) or to set larger allowances for repeatability.
For instance, in order to obtain a repeatability of not more than 0.1% it is necessary to conduct 10 proving passes. Furthermore, increasing the number of proving passes increases the flowmeter's repeatability and also improves the quality of the mean's value.
However, there is no description of the relationship between a number of proving passes, the required repeatability and the mete pulse variations.